feat(commits): add Conventional Commits message normalizer

[iamlasse]

Summary

Adds a Conventional Commits message normalizer to the nightshift CLI, addressing all review feedback from PR #207 (iteration 2).

• internal/commits — pure Normalize/validate package enforcing the project's rules: known lowercase type set (feat fix docs style refactor test chore perf build ci revert), optional (scope), breaking-change ! marker, lowercase non-capitalized subject, rune-based 72-char subject limit, git-comment stripping, whitespace/trailing-period cleanup, and 72-rune body wrapping that preserves paragraph breaks. Idempotent. Errors are wrapped, errors.Is-matchable sentinels.
• CLI — nightshift commit normalize accepting a positional message, --file, or stdin, plus --check for validate-only. Registered on the root command.
• scripts/commit-msg.sh — git commit-msg hook that normalizes the message in place and rejects invalid messages with a friendly diagnostic block.
• docs/commit-messages.md — format rules, type list, command examples, and hook installation.

Fixes from iteration-1 review

1. --check is now truly validate-only — it no longer prints the rewritten message; it prints nothing on success and exits non-zero on failure.
2. Hook diagnostic block is no longer dead code — NORMALIZED="$(...)" || STATUS=$? tolerates the failing command substitution under set -euo pipefail, so the expected-format/types/--no-verify guidance now actually prints.
3. Single clean error output — SilenceUsage/SilenceErrors on the command; no more Error: + full Usage dump on every validation failure.
4. No temp-file leak — hook uses mktemp (cleaned via a trap) instead of a fixed /tmp/nightshift-commit-msg.err.
5. Rune-consistent wrapping — body wrapping now measures width in runes, matching the rune-based subject limit (verified for multibyte text).
6. Renamed sentinel — ErrSubjectLowercase → ErrSubjectCapitalized (matches its "must not be capitalized" message).
7. feat! breaking-change support — feat!: / feat(scope)!: parse and round-trip instead of producing a confusing "unknown type" error; revert added to the allowed type set.

Verification

• go build ./..., go vet ./..., gofmt -l . — clean
• go test ./internal/commits/... — pass
• CLI exercised: --check validate-only, single-line error output, rewrite mode, breaking-change output
• Hook exercised end-to-end: bad message prints diagnostics + exits 1; good message rewrites in place + exits 0; no temp leak

Nightshift-Task: commit-normalize
Nightshift-Ref: https://github.com/marcus/nightshift